1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a transflective liquid crystal display device selectively using reflective and transmissive modes and having equivalent light efficiency in reflective and transmissive portions with high brightness resulting from high aperture ratio, and a fabricating method thereof.
2. Discussion of the Related Art
Generally, transflective liquid crystal display (LCD) devices function as both transmissive and reflective LCD devices. Because transflective LCD devices can use both a backlight and natural or artificial ambient light, the transflective LCD devices may be used in more circumstances, and power consumption of transflective LCD devices may be reduced.
FIG. 1 is an exploded perspective view of a liquid crystal display (LCD) device according to the related art. As shown in FIG. 1, a liquid crystal display (LCD) device 10 has an upper substrate 80 having a black matrix 84, a color filter layer 82 including sub-color filters and a common electrode 86 on the color filter layer 82, and a lower substrate 60 having a thin film transistor (TFT) “T” and a pixel electrode 66 connected to the TFT “T.” A liquid crystal layer 95 is interposed between the upper and lower substrates 80 and 60. The lower substrate 60 is referred to as an array substrate because array lines including a gate line 61 and a data line 62 are formed thereon. The gate line 61 and the data line 62 cross each other forming a matrix, and the TFT “T” of a switching element is connected to the gate line 61 and the data line 62. The gate line 61 and the data line 62 define a pixel region “P” by crossing each other, and the TFT “T” is formed near a crossing portion of the gate line 61 and the data line 62. The pixel electrode 66 is formed of a transparent conductive material such as indium-tin-oxide (ITO) and indium-zinc-oxide (IZO) in the pixel region “P.” The upper substrate 80 is referred to as a color filter substrate because the color filter layer 82 is formed thereon.
A reflective electrode 64 of a reflective material such as aluminum (Al) or aluminum alloy is formed in the pixel region “P.” The reflective electrode 64 has a transmissive hole “H” so that the pixel region “P” is divided into a reflective portion “C” and a transmissive portion “D.” The transmissive portion “D” corresponds to the transmissive hole “H” and the reflective portion “C” corresponds to the reflective electrode 64.
FIGS. 2 and 3 are schematic cross-sectional views, which are taken along a line “II-II” of FIG. 1, showing a transflective liquid crystal display device according to first and second embodiments of the related art, respectively.
In FIG. 2, first and second substrates 60 and 80 face into and are spaced apart from each other and a liquid crystal layer 95 is interposed therebetween. The first and second substrates 60 and 80 include a plurality of pixel regions “P.” A gate line (not shown) and a data 62 line crossing each other are formed on an inner surface of the first substrate 60. A color filter layer 82 including a red sub-color filter (not shown), a green sub-color filter 82a and a blue sub-color filter 82b is formed on an inner surface of the second substrate 80, and a black matrix 84 is formed between the sub-color filters 82a and 82b. A transparent common electrode 86 is formed on the color filter layer 82 and the black matrix 84.
The pixel region “P” may be divided into a reflective portion “C” and a transmissive portion “D.” Generally, a transparent electrode 66 corresponding to the pixel region “P” is formed over an inner surface of the first substrate 60. A reflective electrode 64 having a transmissive hole “H” can be formed over or under the transparent electrode 66. The transmissive hole “H” corresponds to the transmissive portion “D” and the reflective electrode 64 corresponds to the reflective portion “C.”
In a transflective LCD device, it is very important to obtain an equivalent optical efficiency and color reproducibility in the reflective and transmissive portions “C” and “D.” In the reflective portion “C,” light passes through the color filter layer 82 and the liquid crystal layer 95, and then reflects from the reflective electrode 64. The light reflecting from the reflective electrode 64 passes through the liquid crystal layer 95 and the color filter layer 82 again, and then is emitted to exterior. Accordingly, the light passes through the color filter layer 82 and the liquid crystal layer 95 having a thickness (i.e., a cell gap) “d” twice in the reflective portion “C.” Because a first light path (a distance that light transverses) in the reflective portion “C” is twice that of a second light path in the transmissive portion “D,” a first retardation value of 2d·Δn (n is a refractive index of the liquid crystal layer 95) in the reflective portion “C” is twice of a second retardation value of d·Δn in the transmissive portion “D.” As a result, an equivalent optical efficiency is not obtained in the reflective and transmissive portions “C” and “D.” To solve this problem, as shown in FIG. 3, a transflective LCD device having a cell gap ratio of 2d:d in transmissive and reflective portions has been suggested.
In FIG. 3, first and second substrates 60 and 80 face into and are spaced apart from each other and a liquid crystal layer 95 is interposed therebetween. The first and second substrates 60 and 80 include a plurality of pixel regions “P.” A gate line (not shown) and a data 62 line crossing each other are formed on an inner surface of the first substrate 60. A color filter layer 82 including a red sub-color filter (not shown), a green sub-color filter 82a and a blue sub-color filter 82b is formed on an inner surface of the second substrate 80, and a black matrix 84 is formed between the sub-color filters 82a and 82b. A transparent common electrode 86 is formed on the color filter layer 82 and the black matrix 84.
The pixel region “P” may be divided into a reflective portion “C” and a transmissive portion “D.” Generally, a transparent electrode 66 corresponding to the pixel region “P” is formed over an inner surface of the first substrate 60. A reflective electrode 64 having a transmissive hole “H” can be formed over or under the transparent electrode 66. The transmissive hole “H” corresponds to the transmissive portion “D” and the reflective electrode 64 corresponds to the reflective portion “C.”
An insulating layer 63 having an opening 61 is formed under the reflective electrode 64. The opening 61 corresponds to the transmissive portion “D.” The liquid crystal layer 95 is formed to have a thickness ratio (cell gap ratio) of 2d:d in the transmissive and reflective portions due to the opening 61, thereby an equivalent retardation value of 2d·Δn results in both in the reflective and transmissive portions “C” and “D.”
However, the transflective LCD device of FIG. 3 has a disadvantage such as a disclination at a border of the reflective and transmissive portions “C” and “D.” The disclination is illustrated in FIGS. 4 and 5.
FIG. 4 is a schematic plane view showing a pixel region of an array substrate for a transflective liquid crystal display device according to a second embodiment of the related art and FIG. 5 is a schematic cross-sectional view taken along a line “V-V” of FIG. 4.
In FIG. 4, a gate line 61 and a data line 62 crossing each other are formed on a substrate 60. An intersection of the gate line 61 and the data line 62 defines a pixel region “P”. A thin film transistor (TFT) “T,” including a gate electrode 70, an active layer 72, a source electrode 74 and a drain electrode 76, is formed at the intersection of the gate line 61 and the data line 62. The pixel region “P” is divided into a reflective portion “C” and a transmissive portion “D.” A transparent electrode 66 is formed to correspond to the pixel region “P” and a reflective electrode 64 is formed to correspond to the reflective portion “C.” The reflective electrode 64 has a transmissive hole corresponding to the transmissive portion “D.” To obtain an equivalent optical efficiency in the reflective and transmissive portions “C” and “D,” an insulating layer (not shown) is formed under the reflective electrode 64 to have a opening (not shown) corresponding to the transmissive portion “D.” Accordingly, a step is generated at a border region “F” of the reflective and transmissive portions “C” and “D” and the step causes an incline.
In FIG. 5, a border region “F,” where an incline is observed, includes a first width “F1” including a slanted surface of an insulating layer 63 and a second width “F2” extending from the slanted surface. The insulating layer 63 has a thickness of t and the slanted surface has an angle of θ with respect to a top surface of the substrate 60. When the thickness is about 2 μm and the angle is about 50°, the first width “F1” may be calculated from F1=t/tan θ≈1.7 μm. Because the second width “F2” is generally about 1.5 μm, a total width of the border region “F” is about 3.2 μm.
Referring again to FIG. 4, a total area “A” of the border region “F” may be calculated from A≈2×(L+W)×3.2 μm2, where L and W are a length and a width of the transmissive portion “D.” As the transmissive portion “D” increases, the border region “F” where an incline is observed increases. Accordingly, the aperture ratio is degraded by about 10% and the degradation of aperture ratio causes a reduction in contrast ratio.